1. The present invention relates to an abrasive composition. More particularly, the present invention relates to an abrasive composition for rapidly polishing and providing a mirror finish to an aluminum-based substrate for a magnetic recording disc, and a process therefor.
2. Description of the Related Art
A magnetic recording disc comprises an aluminum-based substrate including an aluminum or aluminum alloy (referred to only as aluminum hereafter) disc, a nickel substrate formed by plating nickel-phosphine onto an aluminum disc, and an anodized aluminum substrate formed by anodizing an aluminum disc. A magnetic film is applied to the aluminum-based substrate by coating, plating or sputtering, and the surface of the aluminum-based substrate is provided with a mirror-finish by polishing or lapping.
A known weak acid abrasive composition used for polishing an aluminum substrate for a magnetic recording disc is an aqueous slurry containing an aluminous abrasive and an abrasion accelerator of nickel sulfate (H. Kitano et al., Japanese Unexamined Patent Publication (Kokai) No. 61-278587, published on Dec. 9, 1986). A weak acid to neutral abrasive composition is also provided, which is an aqueous slurry containing an aluminum abrasive and nickel sulfamate (Japanese Pat. Application No. 62-253591, field on Oct. 9, 1987).
Due to recent demands for a denser recording on a magnetic recording disc, the alumina-based substrate requires a finer polished surface with fewer pits, protrusions and scratches, but the above abrasive compositions may easily cause protrusions or pits and deep scratches, particularly when polishing an aluminum substrate plated with nickel-phosphine, under normal conditions.
Therefore, the main object of the present invention is to provide an abrasive composition by which a finer mirror-like surface with few protrusions or pits and few deep scratches is obtained at a high efficiency.